Ok it looks like Luke has confirmed this is a filtering based on the OP_RETURN byte limit. So it’s not specifically filtering coinjoins, they just happen to be over the 42 byte limits for whatever reason.

Both decisions seem odd to me, Luke says there’s no reason for using that many bytes, Samourai says the byte limit has been 80 for ages (which it has).

So now I’m curious what that data is for in this scenario. And why either side cares about this. Seems like both have the capacity to fix this if the excessive OP_RETURN data isn’t even necessary. Also curious if this is something that specifically distinguishes whirlpool transactions from other types (which there might be numerous other ways that this happens), just because it would seem beneficial to look as inconspicuous as possible rather than stand out in whatever way their use of that may imply.

In other words, this seems like something either party could fix potentially but knowing how stubborn they both are, it might not change.